In electrophotography, a photoconductor is charged and then exposed imagewise to light. In the area of the photoconductor exposed to light, the charge dissipates or decays while the dark areas retain the electrostatic charge.
The difference in the charge levels between the areas exposed to light and the dark areas produces electrical fields therebetween. Accordingly, the resultant latent electrostatic image on the photoconductor is developed by depositing small colored particles, which are known as toner particles, over the surface of the photoconductor with the toner particles having a charge so as to be directed by the electrical fields to the image areas of the photoconductor to develop the electrostatic image.
A number of means are known for developing the latent electrostatic image by the application of the toner particles. One of these is known as cascade development and is described in U.S. Pat. No. 2,618,552 to Wise.
Another means is known as the magnetic brush process. This method is described in U.S. Pat. No. 2,874,063 to Greig.
In each of the cascade and magnetic brush development processes, a two component developer material is utilized. The developer material comprises a mixture of small toner particles and relatively large carrier particles. The toner particles are held on the surfaces of the relatively large carrier particles by electrostatic forces which develop from the contact between the toner and carrier particles producing triboelectric charging of the toner and the carrier to opposite polarities. When the developer material is moved into contact with the latent electrostatic image of the photoconductor, the toner particles are attracted to the latent image.
The toner and carrier particles of the developer material are specially made and processed so that the toner obtains the correct charge polarity and magnitude of charge to insure that the toner particles are preferentially attracted to the desired image areas of the photoconductor. For a given developer-hardware system, the magnitude of the triboelectric charge is important in that if such charge is too low, the copy will be characterized by high print density but heavy background and poor resolution; if the charge is too high, the background and resolution if good but the print density will tend to be low. Thus, there is an optimum range of toner charge for best overall results.
Prior art dry developer materials, which are employed in automatic copy machines, have carrier filming problems due to the recycling of the carrier particles through many cycles which produces many collisions between the carrier particles and between the carrier particles and parts of the machine. The attendant mechanical friction causes some toner material to form a physically adherent film on the surfaces of the coatings of the carrier particles.
When this occurs, there is a gradual accumulation of a permanently attached film of toner material on the surfaces of the carrier particles. In such a film, the toner appears to lose its original particulate shape and form plaques that are tenaciously adherent to the carrier bead surface. This film of toner impairs the normal triboelectric charging of the toner particles in the developer mix because the normal toner-carrier triboelectric charging is partly replaced by a toner-toner relationship. As a result, the toner which is available for developing the latent electrostatic image is less highly charged on the average. If this occurs to a sufficient degree, the improperly charged toner particles can be deposited on the non-image areas whereby the quality of the copies is impaired since the non-image areas possess an unacceptable level of background toner.
When toner filming occurs to a sufficient degree, the entire developer material must be replaced thereby increasing the cost of operation of the copy machine. Furthermore, it is time consuming. This problem is especially significant in high speed copy machines in which thousands of copy cycles occur in a relatively short period of time or in copy machines in which the developer is continuously agitated.
The toner filming mode of failure has been observed as the dominant mode of failure of carrier beads in the past, as it manifested itself relatively early in the life of the developer mix and for otherwise good carrier coatings the other modes did not occur at this early point in the carrier life. Spalling and abrasive wear out did not generally cause the carrier to fail as these mechanisms were slower. Only after the toner filming failure was at least partially overcome did the other failure modes become significant problems to be addressed. Only at this point did material properties possibly affecting these failures become of prime interest and efforts were made to identify properties which contribute to the solution of these failure problems.
Furthermore, because of the contact between the carrier particles and between the carrier particles and parts of the machine, there is abrasion of the coating of the carrier particles. This abrasion of the coating also may reduce the effectiveness of the triboelectric charging between the carrier and the toner by effectively exposing the toner to the core material of the carrier, if the abrasive wear proceeds far enough.
Thus, if the coating is not sufficiently resistant to abrasion, early replacement of the entire developer material is required. Again, this replacement of the entire developer material is costly and time consuming, especially in high speed copy machines.
Furthermore, even if the coating of the carrier particle resists abrasion, the coating also must have good adhesion to the core of the carrier particle in order to prevent loss of the coating by fracture at the core-coating interface. Otherwise, the coating can chip, flake or spall due to the rubbing or contact between the various carrier particles and between the carrier particles and parts of the machine. This also requires early replacement of the developer material.
In addition to having the foregoing desired properties, the carrier particles must have the characteristic of having a triboelectric charge of a desired magnitude and polarity when used with a particular electroscopic toner. This is because the magnitude of the triboelectric charge of the toner is controlled by the magnitude and polarity of the carrier charge when the toner and the carrier are mixed.
The magnitude of the charge of the toner is important for the electrophotographic system to produce copies of an excellent quality. If the magnitude of the charge of the toner is too low, the images have poor edge definition and lack contrast since the non-image or background areas possess an unacceptable level of toner as previously mentioned. If the magnitude of the charge of the toner is too high, then there is poor image fill since the toner would tend to stay with the carrier. Accordingly, the magnitude of the charge of the toner must be within a range above that in which poor edge definition is produced and below that in which poor image fill occurs.
Thus, if the coating of the carrier particle has the characteristic of imparting a triboelectric charge to the toner when mixed therewith so that the toner charge does not fall within the range in which copies of high quality can be obtained, the coating cannot be employed for a carrier for the toner even though it meets the abrasion, anit-stick, and adhesion requirements. Accordingly, for a carrier coating to be satisfactory, it not only must meet the abrasion, anti-stick, and adhesion requirements but also must be capable of triboelectrically charging the toner to a level within a desired range that enables copies of high quality to be produced.
Carrier failure modes have heretofore been rather incompletely understood. As an example of the lack of understanding of carrier failure modes, the term "tired iron" has been used to describe failure of a steel or iron filing carrier to triboelectrically charge toner after it has been used as a carrier for some time. The primary modes of failure which occur in a developer mix are filmed-on toner, spalling, and abrasive wear. Heretofore, filmed-on toner has been the dominant mode of early carrier failure. The carrier beads acquire a thin film of fused toner material and thus triboelectrically fail long before the coatings on the beads have spalled from the cores. When a carrier coating is discovered which inhibits the film-on toner failure, the failure mode shifts to spalling. If the coating also resists the spalling failure then the next progressive mode of failure is abrasive or adhesive wear of the triboelectric carrier coating.
The presently known requirements of a carrier coating material other than a triboelectric relationship are thus dictated by these failure modes. Low surface energy is a very substantial consideration when attempting to overcome the dominant failure mode of carrier materials -- filmed-on toner.
The adhesion qualities of a particular resin are pertinent when filmed-on toner is not a problem and the failure mode becomes that of spalling of the carrier coating from the carrier bead. When a material is found which provides adequate resistance to the failure modes of filmed-on toner and spalling, the abrasive or adhesive wear characteristics of that material then become a primary consideration in attempting to lengthen the life of the carrier material. As can be seen from the foregoing, the understanding of carrier failures has developed only as carrier materials have been selected and as they have been forced to a failure point. Therefore, the criteria for materials which can be used for carrier coatings to impart triboelectric charge to toner particles, have been developed through failure in a trial and error type process.
Otherwise useful and high quality carriers suffer greatly degraded useful lives when used in hostile developer environments. Examples of such hostile environments are development electrode cascade developer apparatus and magnetic brush developer apparatus.
One example of the degree of degradation of carrier life in a development electrode developer assembly is an automatic electrostatic copier which is commercially available with or without a development electrode. When that copier is constructed with a cascade developer without a development electrode the published life of the charge of carrier is 300,000 copies between carrier replacement. When a development electrode is installed in the same model machine the same carrier charge must be replaced after only 100,000 copies. This decrease of about 66% in the useful life of the carrier is ample evidence of the hostility of development electrodes in cascade developers. These machines use 25 pound charges of carrier, thus producing only 4,000 copies per pound of carrier when using a development electrode.
Development of latent electrostatic images by magnetic brush development has proved to be a substantial problem ever since the original technique was disclosed in U.S. Pat. No. 2,847,063 to Greig, as referenced above. It has long been recognized that the magnetic brush technique of developing will produce very superior copy quality and also to some extent that it removes restrictions on speed of the copy process placed thereby the cascade or powder cloud development techniques. There have also been substantial technological and economic barriers which heretofore have prevented substantial penetration in the market of a commercially acceptable magnetic brush developer unit for a plain paper copier. The primary reason for the technological and economic barriers has heretofore been that the violent agitation of the developer mixture of carrier beads and toner particles has created localized heating and the filming of toner onto the carrier particles much more rapidly and much more detrimentally to the process, than was experienced in cascade development. An additional reason for the apparent failure of the carrier may possibly be explained by the great weight and density of the magnetic core material such as iron grit and steel shot. The substantially greater weight and density of these materials results in increased impact of the carrier beads into one another, the photoconductor, the developer assembly and mechanisms, and the equipment necessary to carry the steel carrier beads to the development zone. This impacting appears to cause some localized heating which then promotes the filming of the toner particles carried by the carrier beads and further results in faster failure of the carrier beads.
In a magnetic brush developer the magnetic carrier beads are formed into a magnetic brush of tendrals of carrier beads and toner particles extending outwardly through the magnetic field of force. This brush vigorously scrubs, at relatively high speed, these tendrals against the photoconductive member bearing the latent electrostatic image. There is, of necessity, some violent scrubbing, agitation and impacting of the carrier beads and the toner particles. Additionally, the developer mix must be agitated, mixed or blended in order to assure a uniform developer mix throughout the operation of the machine. Solutions to this blending problem have been usually solved with continuous mechanical blenders such as augers or paddle blades which continually stir the developer mix to assure uniformity. All of this action contributes to the potential failure of the carrier by filmed on toner. Due to the violent agitation, the coating material on the steel shot (or magnetic core) must additionally adhere to the core with great tenacity to prevent the coating material from chipping, flaking, or spalling off of the core material.
Until very recently, there has been no commercially available plain paper copier using electrostatic copying in the domestic United States market which embodied a magnetic brush developing unit. Recently one new copier product has been marketed which embodies a magnetic brush apparatus. One charge of carrier beads for this developer mix must be replaced after approximately every 10,000 copies, yielding a life on the order of 1,000 to 3,000 copies per pound of carrier. Relatively short carrier life has heretofore made such a developer mix and mechanism lack economic feasibility in the commercial marketplace.
As a basis for comparison, the life of the carrier beads utilized in one commercially available electrostatic plain paper copier utilizing a cascade development system, has been rated as high as 750,000 copies per a 7 pound charge of carrier. When gauged against the life of the carrier in a cascade developer apparatus, carrier beads for magnetic brush developers have a short life and thus increase the operating cost of such a machine by a large factor. This increases the ulitmate cost per copy to the customer, and thus the process has heretofore had economic disadvantages.